In the manufacture of semiconductor devices, metal contacts are deposited onto silicon wafers that have been processed to form devices connected to each other by the metal contacts to form an integrated circuit. In particular, aluminum or aluminum alloy lines have been deposited into vias and openings such as trenches or grooves, formed in a layer such as silicon oxide. This method has long been used and is effective. However, as devices have been made smaller, openings must also be made smaller, and the openings to be filled have higher aspect ratios, i.e., the depth of an opening divided by its width is higher.
Difficulties have been encountered in depositing aluminum lines and contacts by conventional sputtering processes when submicron high aspect ratio openings are to be filled. As the openings become smaller and deeper, the bottom and sides of an opening receive fewer deposited metal ions than the top surface of the device, with the result that the metal layer hangs over the opening. Thus, over time, the bottom and sides of the opening receive even fewer deposited metal ions, and the overhang closes before the opening is filled, creating a void in the opening. This is illustrated in FIG. 1 wherein a deposited aluminum layer 3 overlies and overhangs an opening 5 in a silicon oxide layer 4.
Various techniques have been tried for depositing aluminum-containing layers in an attempt to improve step coverage. For example, combinations of varying sputtering rates and substrate temperatures have been tried in an effort to combine sputtering and melting of the aluminum-containing film so that the aluminum will at least partially melt and flow, thereby forming a continuous film in the opening. However, very high temperatures can damage devices already formed in the substrate or cause changes in other portions of the semiconductor devices in the substrate.
Sputtering chambers have been modified to reduce sputtering rates to allow aluminum atoms to move around and coalesce, but this also increases the time required to make a semiconductor device, which is costly. Further, such modification of existing equipment is also expensive.
Talieh et al in copending application Ser. No. 07/749,096 filed Aug. 23, 1991, "Material Deposition Method for Integrated Circuit Manufacturing", now U.S. Pat. No. 5,171,412 describes a two step sputter deposition process whereby in a first step a metal is deposited at low temperatures (up to about 100.degree. C.) using a collimation grating to improve the directionality of sputtered ions and cover the bottom of openings and grooves in a substrate with a seed metal layer, and then sputter depositing the material without a collimation grating at high temperatures that will allow the metal to flow somewhat, thereby filling the openings and producing a planarized metal layer.
Another advance in the art of sputtering high aspect ratio vias and contacts was disclosed by Inoue et al. Their method comprises depositing aluminum films by sputtering at elevated temperatures. Aluminum/silicon films were deposited by magnetron sputtering. Vias etched into patterned 1 micron thick phosphosilicate glass layers were filled in and the temperature of the substrate maintained at about 530.degree. C. Good step coverage and planarization was achieved. Reactive sputtering of a TiN barrier film about 0.1 micron thick further improved planarization of the above aluminum layer, probably by minimizing reaction between aluminum and the underlying silicon or silicon oxide and it prevented spiking of the aluminum into the silicon substrate.
More recently, a process was described by Chen that specifically addresses sputter deposition and planarization of aluminum for submicron openings. After formation of vertical wall contact openings in a borophosphosilicate glass layer, a first titanium nitride (TIN) barrier layer was deposited in the opening by reactive sputtering. An aluminum alloy layer (AlSi.sub.1% Cu.sub.0.5%) was sputter deposited to fill the openings at an elevated wafer temperature using conventional sputtering equipment. The deposition of a first TiN barrier layer allowed smooth, planarized aluminum films to be deposited while completely filling high aspect ratio openings.
However, none of the above processes is completely effective to fill in submicron size vias and openings of high aspect ratio without the formation of voids and with planarization of the metal layers, in an economically viable process.